The present invention relates to the extrusion of a tubular item or a circular array of filamentous items from any material formable by extrusion. Thus it covers the extrusion of a great variety of end products, such as e.g. tubular film, pipes, twine, rope material and textile yarn from thermoplastic polymers, but also e.g. extruded foodstuff, glass and ceramic articles.
An object of the invention is to achieve improved circumferential distribution in the extrusion of tubular structures or circular arrays of filaments or the like, in particular in connection with materials which due to great deviation from Newtonian behaviour are difficult to distribute evenly over the circumference by conventional means, or in connection with coextrusion.
As known it is relatively easy to carry out an even coextrusion of e.g. 4-5 layers in a flat die, because the spacing of each of the internal slots for the different materials can be adjusted at small intervals, but similar simple adjustment means do not exist for tubular co-extrusion. Therefore there is a great need for devising an improved distribution system in connection with circular coextrusion.
A second object of the present invention is to provide for a simple method and device for coextrusion of a relatively large number of layers.
A third object is to mechanically improve the bonding between layers of different coextruded materials by forming fine ribformed mutually interspersed extensions of the layers.
Other objects of the invention wll appear from the following description.